1. Field
Aspects of the present disclosure relate generally to wireless communication systems, and more particularly, to methods and apparatuses for enhanced received signal processing according to a data-signal-aided channel impulse response (CIR) estimate.
2. Background
Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. One example of such a network is the UMTS Terrestrial Radio Access Network (UTRAN). The UTRAN is the radio access network (RAN) defined as a part of the Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP). The UMTS, which is the successor to Global System for Mobile Communications (GSM) technologies, currently supports various air interface standards, such as Wideband-Code Division Multiple Access (WCDMA), Time Division-Code Division Multiple Access (TD-CDMA), and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). The UMTS also supports enhanced 3G data communications protocols, such as High Speed Packet Access (HSPA), which provides higher data transfer speeds and capacity to associated UMTS networks. As the demand for mobile broadband access continues to increase, research and development continue to advance the UMTS technologies not only to meet the growing demand for mobile broadband access, but to advance and enhance the user experience with mobile communications.
For example, equalization and interference cancellation are key aspects of any WCDMA downlink receiver, such as a user equipment (UE) or a receiver component in such a UE. Due to the large bandwidth used for WCDMA communications, the frequency-selective behavior of a wireless channel is non-negligible and should be combated at the receiver using equalization techniques. In addition, due to reuse of carrier frequencies in adjacent cells, a WCDMA downlink signal can be significantly impaired by inter-cell interference, which is often combated using interference rejection and/or interference cancellation techniques.
Certain legacy solutions to these issues rely heavily on an estimate of the wireless channel impulse response (CIR), not only for equalization, but also for interference estimation/cancellation. In these legacy solutions, the CIR is estimated from a known pilot or Common Pilot Channel (CPICH) transmitted by a network entity (e.g., a NodeB and/or base station) associated with a cell.
Some sample techniques used in such legacy solutions utilize the following parameters and functions in computing estimated CIRs:                x(n): Signal, e.g., sequence of chips, transmitted by the network entity        h(k): kth tap of a wireless channel from the network entity to the UE        y(n): Sequence of chips received at the UE        w(n): Additive thermal noise/interference from other cells received at the UE        
In such legacy solutions, the UE receiver may first receive a signal from the network entity. This received signal may be represented at the chip level in terms of the above notation by the function:
      y    ⁡          (      n      )        =                    ∑                  k          =          0                          K          -          1                    ⁢                          ⁢                        h          ⁡                      (            k            )                          ⁢                  x          ⁡                      (                          n              -              k                        )                                +          w      ⁡              (        n        )            wherein K is the length of channel, for example, in chips.
Furthermore, the signal contains a known pilot signal and an unknown data signal, both transmitted by the network entity, and may be represented (as transmitted) by the function:
      x    ⁡          (      n      )        =                                          E            cp                                I            0                              ⁢                        o          0                ⁡                  (          n          )                    ⁢              x        0              +                  ∑                  j          =          1                255            ⁢                          ⁢                                                  E              cj                                      I              0                                      ⁢                              o            j                    ⁡                      (            n            )                          ⁢                  x          j                    Here I0 represents the received signal power, Ecp represents the pilot channel power, and Ecj represents the power of the jth channel, xj denotes the symbol transmitted on the jth channel, oj(n) is the product of the orthogonal variable spreading factor (OVSF) code for channel j and the scrambling code sequence for the cell, and j=0 (associated with the first term of the function) represents the pilot (and/or CPICH) channel.
Furthermore, the correlation of y(n) and o0(n)x0 (i.e. the pilot channel) is represented in some legacy solutions as:
      E    ⁡          [                        y          ⁡                      (                          n              +              l                        )                          ⁢                              o            0                    ⁡                      (            n            )                          ⁢                  x          0          *                    ]        =      E    ⁡          [                                    ∑                          k              =              0                                      K              -              1                                ⁢                                          ⁢                                    h              ⁡                              (                k                )                                      ⁢                          x              ⁡                              (                                  n                  +                  l                  -                  k                                )                                      ⁢                          {                                                                    o                    0                    *                                    ⁡                                      (                    n                    )                                                  ⁢                                  x                  0                  *                                            }                                      +                              w            ⁡                          (                              n                +                l                            )                                ⁢                      {                                                            o                  0                  *                                ⁡                                  (                  n                  )                                            ⁢                              x                0                *                                      }                              ]      where any asterisk notation (*) donates the complex conjugate of a quantity, and where l is an index associated with the correlation. In addition, assuming that any noise or interference and the transmitted chips are not correlated and that the OVSF code associated with the pilot channel is orthogonal to the OVSF codes associated with other channels, legacy methods may update the previous function to:
      E    ⁡          [                        y          ⁡                      (                          n              +              l                        )                          ⁢                              o            0                    ⁡                      (            n            )                          ⁢                  x          0          *                    ]        =                    ∑                  k          =          0                          K          -          1                    ⁢                          ⁢                        h          ⁡                      (            k            )                          ⁢                  E          ⁡                      [                                          {                                                                                                    E                        cp                                                                    I                        0                                                                              ⁢                                                            o                      0                      *                                        ⁡                                          (                                              n                        +                        l                        -                        k                                            )                                                        ⁢                                      x                    0                    *                                                  }                            ⁢                              {                                                      o                    0                    *                                    ⁢                                      (                    n                    )                                    ⁢                                      x                    0                    *                                                  }                                      ]                                +    0  
Additionally, because WCDMA chips are scrambled using relatively long pseudo-noise (PN) codes, the transmitted chips are statistically uncorrelated with each other, such that E[o0(m)o0*(n)]=δ(m−n), wherein δ is a delta function equal to 1 when m=n, otherwise the delta function equals 0. Using this property, legacy UEs can reduce the above function to:
      E    ⁡          [                        y          ⁡                      (                          n              +              l                        )                          ⁢                              o            0                    ⁡                      (            n            )                          ⁢                  x          0          *                    ]        =                              E          cp                          I          0                      ⁢                  ∑                  k          =          0                          K          -          1                    ⁢                        h          ⁡                      (            k            )                          ⁢                  δ          ⁡                      (                          l              -              k                        )                              
Thus, according to legacy solutions, the cross-correlation of the pilot signal and y(n) yields an estimated CIR:
      E    ⁡          [                        y          ⁡                      (                          n              +              l                        )                          ⁢                              o            0                    ⁡                      (            n            )                          ⁢                  x          0          *                    ]        =                              E          cp                          I          0                      ⁢          h      ⁡              (        l        )            
Therefore, in some legacy solutions, the CIR is estimated by correlating the received signal with the chips transmitted on the pilot channel (e.g., CPICH). This is possible because the OVSF code used with the pilot channel is known by the UE and the other channels are transmitted using orthogonal OVSF codes. Thus, the received signal can be utilized to compute the estimated CIR.
In typical WCDMA deployments, the pilot signal is transmitted constantly on the pilot channel by the base station or NodeB at a fraction of the total cell power—often as little as ten percent. This limits the quality of the estimated CIR at the UE and impacts end user performance as a result. Therefore, there is a need for methods and apparatuses for improved CIR estimation to provide more reliable channel estimates upon which received signal equalization, interference estimation, and/or interference cancellation may be based.